Chemical safety checks need to be tightened up around the world to protect children from a "silent epidemic" of brain disorders, say experts.

In the past seven years, the number of recognised chemical causes of brain development conditions has doubled from six to 12, two leading scientists who conducted a review of study evidence found.

The list of chemicals known to damage the human brain but not regulated to safeguard children had also risen from 202 to 214.

These hazardous substances are found in everyday items including clothing, furniture and toys, the experts point out.

They are calling for universal legal requirements forcing manufacturers to prove that all existing and new industrial chemicals are non-toxic before they reach the market place.

In the EU, the Reach (Registration, Evaluation, Authorisation and Restriction of Chemicals) regulations already impose such rules.

Without them being applied globally, the world faces a "pandemic of neurodevelopmental toxicity", said Dr Philippe Grandjean, one of the study authors from Harvard School of Public Health in Boston, US.

Neurodevelopmental disorders such as autism, attention deficit hyperactivity disorder (ADHD), dyslexia and cerebral palsy affect one in six children worldwide.

Growing evidence strongly links these conditions to childhood exposure to hazardous chemicals such as mercury, lead, solvents and pesticides, say the scientists writing in the journal The Lancet Neurology.

Dr Grandjean and co-author Dr Philip Landrigan from Mount Sinai School of Medicine in New York believe this is only the tip of the iceberg.

The vast majority of the more than 80,000 industrial chemicals in widespread use in the US have never been tested for their toxic effects on the developing foetus or child, they argue.

"The only way to reduce toxic contamination is to ensure mandatory developmental neurotoxicity testing of existing and new chemicals before they come into the marketplace", said Dr Landrigan. "Such a precautionary approach would mean that early indications of a potentially serious toxic effect would lead to strong regulations, which could be relaxed should subsequent evidence show less harm."

A new international prevention strategy is needed that places the burden of responsibility on chemical producers rather than governments, say the experts.

They conclude: "The total number of neurotoxic substances now recognised almost certainly represents an underestimate of the true number of developmental neurotoxicants that have been released into the global environment.

"Our very great concern is that children worldwide are being exposed to unrecognised toxic chemicals that are silently eroding intelligence, disrupting behaviours, truncating future achievements, and damaging societies, perhaps most seriously in developing countries."

Prof Andy Smith, senior scientist at the Medical Research Council Toxicology Unit in Leicester, cautioned: " The epidemiological studies that this review looked at have reported links between toxicity of synthetic chemicals and brain development differences.

"However, these studies mostly identify associations rather than causal relationships. As usual thousands of chemicals of 'natural' source are not considered.

"The implication that present exposure to minute levels of many thousands of synthetic chemicals, even as mixtures, are strong drivers of highly complex neurological disorders and intelligence should be considered with reservation."